[mod] utils/filtron.sh - check a filtron installation

Impplement a script to check a filtron installation::

   ./utils/filtron.sh install check

Related-to: https://github.com/searxng/searxng/issues/450
Related-to: https://github.com/searxng/searxng/pull/446#issuecomment-954599668



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>